1. Field of Invention
The present invention relates generally to managing energy in plural energy storage units. More particularly, the present invention relates to a method and system for managing energy in plural energy storage units in which individual energy storage units are each monitored and the charge in each unit equalized during charging and/or discharging.
2. Background Information
Battery packs composed of multiple cells can use internal electronics to assist in managing performance and safety issues. Individual cells can be connected in series to provide higher battery pack voltages for high voltage applications (for example, for use in notebook computers and vehicles). Metal hydride cells, such as nickel metal hydride (NiMH), and lithium-based chemistries, such as Li-ion cells, can be damaged or even explode if undercharged or overcharged. Internal electronics can be used for safety reasons to maintain safe voltages, currents and temperatures within the battery pack.
Additional examples of uses for internal electronics in battery packs include gauging charge levels to determine remaining run time or charging time, managing the charge and discharge process, and communicating with a host device or appliance.
Individual cell voltages may not remain balanced (equal) across a series connected grouping of individual cells or group of cells. Rather, some cells can be over potential and other cells can be under potential as referenced to the voltage measured across all the cells. Therefore, batteries and battery packs can include circuitry to monitor individual cells and, for example, stop the current to the batteries or battery packs before damage occurs.
U.S. Pat. No. 5,631,534, the contents of which are herein incorporated by reference, discloses circuitry associated with a battery pack for controlling the voltage balance between cells. A balancing circuit can be used during the charging process to transfer energy from those cells which reach full charge first to those cells that are at a voltage less than full charge.